Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various elastomers.
Some tire treads are of a cap/base construction, with the tread cap designed to be ground-contacting with an outer surface of a lug/groove configuration, and with the tread base underlying and supporting the tread cap and positioned between the tread cap and the tire carcass. The tread base is not intended to be ground-contacting and, thus, not normally intended to have the same measure of tread properties as, for example, the tread cap typically desired properties of traction and treadwear.
While the tread cap, in a tread cap/base construction, is typically designed to be ground-contacting and, therefore, provide traction in combination with acceptable tread wear and rolling resistance, the underlying tread base is typically designed to fulfill an entirely different function and is not designed to be ground-contacting. In particular, it is typically desired that the tread base fulfill a function of transmitting multiaxial tread cap solicitations to the tire carcass, usually desirably with relatively low heat generation. The term "tread cap solicitations" is used herein to mean "the forces resulting from the tread cap working under forces such as compression, bending and/or shear", all of which can cause heat generation and, thus, cause a temperature build-up, and, also cause the forces to impact on the tire carcass itself. Such forces can result, for example, from the tire's cornering, braking and various handling activities, all of which can generate heat build-up within the tire tread.
Accordingly, a tire's rubber tread base component of a tread can fulfill very important functions as an interface between the tread cap and the tire carcass.
Desirable properties for the tread base include, for example, good rubber hysteresis so that it generates less heat from the stresses transmitted by the solicitations of the rubber tread cap, thereby reducing heat build-up in the tire carcass itself. The desirability of a tread base having good hysteresis to reduce heat build-up is well known to those skilled in such art.
Further, by nature of the tread base transmitting and absorbing portion of the stresses transmitted by the solicitations of the tread cap, the tire's overall handling, braking and/or cornering properties can be enhanced.
For example, during tire use operations such as tire handling, braking and/or cornering, the tread base transmits a degree of the stresses from the tire carcass to the tread and, conversely, from the tread to the carcass an optimum stress transmission is a balance between stress transmission and stress absorption by the tread base. It is desirable that such optimum balance be kept relatively constant during a substantial portion of the useful life of the tire, particularly as the tire ages, over a wide range of stress and strain intensity histories for the tire.
In one aspect, tread base rubber compositions may be provided, for example, with various amounts of various fillers intended to promote a relatively high stiffness and relatively low hysteresis, particularly as compared to the tire tread cap itself.
However, it remains to be desirable, for tire steering and cornering tire performance, to provide a tire tread base rubber composition having both low hysteresis and high stiffness characteristics together with a low stiffness variation characteristic for various strain or stress intensity histories. By the term "stress intensity history" is meant herein to be "the maximum stress intensity seen by the tread base during a representative period of time".
By the term "low stiffness variation" it is meant that "the dynamic stiffness of the tread base rubber composition does not change appreciably over a reasonable period of time and after typical strain histories". Such dynamic stiffness can be represented, for example, by shear complex modulus G*, and other resultant viscoelastic properties such as loss modulus G" and tangent delta values, taken at about 60.degree. C.
In practice, for the tire tread of this invention, it is intended that the shear complex modulus G* of the tire tread base composition does not vary more than about 15 percent, preferably not more than ten percent, for the normal useful life of the tire.
Conventional analytical apparatus, or equipment, is used for measuring elastic modulus G*, loss modulus G" and tangent delta values of rubber compositions and the use and application of such apparatus for determining such values for rubber compositions is believed to be well known to those skilled in such rubber composition analytical art.
In practice, it is considered herein to be important to use tire tread base rubber compositions which have relatively stable rubber properties such as, for example, the aforesaid relatively high stiffness and relatively low hysteresis properties during the expected useful life of the tire under typical operating conditions. Thus, it is desired herein to provide such a tread base composition having a relatively low stiffness variation, or softness.
This is considered important because variations of the tire tread base, or undertread, properties such as stiffness and hysteresis during the expected useful life of the tire under typical operating conditions and solicitations of the tire tread will conventionally be expected to result in a drop, or reduction, in tire performance characteristics. For example, the tire's rolling resistance which may increase, thus, increasing vehicular fuel consumption, the tire's handling performance such as cornering may decrease, as well as the tire's high speed performance may decrease.
For example, and as a measure of heat resistance in terms of heat build-up, a blow-out test of the tread base rubber composition may be used to show an advantage of using the rubber composition of this invention for lower heat generation capabilities. The term "blowout" relates to a test which can briefly be described as being conducted by DIN Method No. 53533 with a Goodrich flexometer and is well known to those having skill in such art.
The blow-out test involves subjecting a rubber sample of definite size and shape to high frequency oscillating compressive stresses under controlled conditions. The temperature is measured versus time required for fatigue failure of the sample by internal rupture or blow-out.
In the description of this invention, the term "phr" where used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight or rubber, or elastomer".
In the description of this invention, the terms "rubber" and "elastomer", where used herein unless otherwise prescribed, are used interchangeably. The terms "rubber composition", "compounded rubber" and "rubber compound" where used herein unless otherwise prescribed, are used interchangeably to refer to "rubber which has been blended or mixed with various ingredients or materials" and such terms are well known to those having skill in the rubber mixing, or rubber compounding, art.
The Tg of a polymer, particularly an elastomer, if used herein unless otherwise prescribed, refers to its glass transition temperature which can conventionally be determined, for example, by a differential scanning calorimeter at a heating rate of, for example, about 20.degree. C., to an observed transition of the temperature versus time curve. It is understood that such Tg determination is well known to those having skill in such art.